8/12/2019 saruman field

1/2

significance. In addition, some of the model uncertainty cases include FEPs that could berelevant but are not considered in the Reference Case.

In addition, a number of calculation cases are considered for the perturbation scenarios.Isolation failure scenarios are treated either qualitatively or by less formal "what if?"

calculations.

Figure 2-1 outlines the procedure for evaluating barrier performance and the system safety inH12.

Figure2-1 Procedure for evaluating barrier performance and the system safety

2.3 Modeling strategy

The near-field, consisting of the EBS and a limited volume of the surrounding host rock, arethe elements of a repository system that tend to be characterized by least uncertainty. InH12, this has led to the development and application of relatively realistic near-field datasetsand models, although with moderately conservative assumptions made where there is

uncertainty2-2)

. In the case of the surrounding geosphere, greater uncertainties associated, forexample, with the characterization of large-scale heterogeneity from a limited number of in-situ measurements, lead to a more conservative modeling strategy for this part of the system.Emphasis on the near-field is considered appropriate given the complex geological structureof Japan, especially at the stage prior to site selection.

A different approach is taken for the biosphere assessment. No attempt is made to model theevolution of the surface environment and the lifestyles of future generations, due touncertainties that are largely irreducible. Rather, certain sets of assumptions are made about

2-2) In the case of the near-field host rock, hypothetical datasets have been defined for generic geological

environments, with a level of conservatism that matches the confidence level established through in-situmeasurements and experiments at a number of field test sites.

Evaluation of Barrier Performance Evaluation of System Safety

Reference Case Sensitivity analysis

Construct/compileappropriate

Conceptual models Mathematical models Data

The Reference Caseprovides a baseline forthe derivation andassessment of alternativecases

Uncertainties inscenarios, models anddata

Variations in thegeologicalenvironment anddesign

Safety

standards

Safety criteriaproposed by

foreignregulatory

bodies

100 300Sv y -1

Evaluation of confidence

Results of foreign safetyassessments

Supplementary safetyindicators

Natural analogues

Total system performance

analysis

Combination of uncertainties and

variationsK e yf a c t or s

8/12/2019 saruman field

2/2

these aspects of biosphere modeling, giving rise to stylized representations of the biosphere("Reference Biospheres", e.g. IAEA, 1996; BIOMASS, 1999a; 1999b). These are used toconvert radionuclide fluxes to doses, and thus provide a means to evaluate the radiologicalconsequences of geological disposal under the assumptions of the Reference Biospheres 2-3) .There is a consensus that the radiation dose to individuals, thus calculated, provides a suitable

safety indicator for geological disposal (IAEA, 1989; OECD/NEA, 1991).

Consistent with the AEC Guidelines, calculations are continued up to (and beyond) the timeof maximum radiological consequences (dose), typically up to 10 7 years. It is, however,recognized that uncertainties associated with geological events and other key processesincrease with time and the applicability of models may decrease. This must be borne inmind when interpreting doses calculated far in the future.

2.4 Confidence in the safety assessment

Since safety assessment does not attempt to provide a precise description of the futureevolution of a geological disposal system, it is not necessary to demonstrate completeunderstanding of every detail of every relevant FEP. In H12, emphasis on the near-field(combined with a rather pessimistic representation of the geosphere) makes the findings of theassessment relatively insensitive to the fairly large uncertainties associated with geospherebarrier performance (especially in the complex Japanese tectonic setting). This significantlyeases the task of building confidence in the models and data used in the assessment.

Another important strategy is to use well established physico-chemical theories in developingmodels and datasets whenever possible. This allows a large body of scientific literature to beutilized and referenced, which provides a sound scientific foundation for the reliability of theassessment.

In addition to these general strategies, the specific measures used to build confidence in thereliability of the H12 safety assessment are described in Chapter VIII; e.g. confidence in theapplicability of models, codes and data has been developed through engineering-scaleexperiments at the ENTRY facility, experiments using radionuclides at the QUALITY facility,geoscientific investigations and a review of the international scientific literature.

2-3) Based on the AEC Guidelines, it is assumed that the lifestyles of future generations are the same as at present.In addition, the concept of "critical groups" (ICRP, 1977; 1985) is adopted.